Ok, we finally some of the info we need. SRAM red 11-28 has 11,12,13,14,15,16,17,19,22,25,28. So it must be the 18 or possibly the 20/21 your legs are wishing for.

as mentioned above, shimano’s 6800 ultegra 14/28 solves this issue as it has a straight block from 14-21 and plus a 23 and maintains the 28 granny cog.

This comes at the expense of top end speed, missing the 11-13 cogs, but really this will only come into play descending under power.

Edit: Honestly, eliminating the 11-13 is a really drastic change. Perhaps drop the 28 and go with the ultegra 12-25, which has 12, 13, 14, 15, 16, 17, 18, 19, 21, 23, 25